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STR #3489
| Application: | FLTK Library |
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| Status: | 5 - New |
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| Priority: | 3 - Moderate, e.g. unable to compile the software |
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| Scope: | 3 - Applies to all machines and operating systems |
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| Subsystem: | Image Support |
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| Summary: | alpha blending does not work correctly in Fl_Image.cxx |
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| Version: | 1.3.4 |
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| Created By: | meixner |
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| Assigned To: | Unassigned |
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| Fix Version: | Unassigned |
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| Update Notification: | |
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Trouble Report Files:
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Trouble Report Comments:
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| #1 | meixner
05:44 Aug 20, 2018 |
| The math behind the alpha blending is not correct, as a result colors may be slightly off.
Alpha is in the range 0..255 and the calculation is as follows:
*dstptr++ = (srcg * srca + dstr * dsta) >> 8;
assume srcg==255, srca==255 (->dsta=0)
*dstptr++=(255*255+dstr*0)>>8; // (=>254)
So white becomes light grey although using a completeley opaque color.
To fix the issue in alpha_blend():
- change srca and dsta from unsigned char to unsigned
- add "srca+=srca>>7" after "srca=*srcptr++"
- change "dsta=255-srca" to "dsta=256-srca"
The changes have to be performed for both grayscale + alpha and RGBA. | |
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